1. Field of the Invention
The present invention relates to an electric double-layer capacitor, and more particularly to a large-capacity electric double-layer capacitor having solid polarizable electrodes.
2. Description of the Related Art
As shown in FIG. 1 of the accompanying drawings, a conventional electric double-layer capacitor comprises a cell stack 109 comprising a plurality of basic cells 106, a pair of pressure plates 107, and a pair of terminal electrodes 108.
As shown in FIG. 2 of the accompanying drawings, each of the basic cells 106 comprises a pair of polarizable electrodes 101 disposed in confronting relationship to each other with a porous, electrically non-conductive, ion-permeable separator 104 interposed therebetween, and housed within a frame-shaped gasket frame 105. Collectors 102 are joined respectively to the upper and lower surfaces of the gasket frame 105, closing the openings defined in the upper and lower surfaces thereof. The separator 104 and the polarizable electrodes 101 are thus sealed within the gasket frame 105. The collectors 102 are held against those surfaces of the polarizable electrodes 101 which face away from the separator 104. Together with the separator 104 and the polarizable electrodes 101, an electrolytic solution 103 is sealed in the gasket frame 105. The separator 104 is impregnated with the electrolytic solution 103 thus sealed in the gasket frame 105.
As shown in FIGS. 3A, 3B, and 3C of the accompanying drawings, each of the polarizable electrodes 101 is in the form of a rectangular plate. The polarizable electrodes 101 are made of solid activated carbon mainly comprising a compound of a powder or fibers of activated carbon and a polyacene material.
The collectors 102 are made of rubber or plastics containing electrically conductive carbon. The collectors 102 are attached under pressure to the polarizable electrodes 101.
As shown in FIGS. 4A, 4B, and 4C of the accompanying drawings, each of the gasket frames 105 is in the form of a hollow frame with openings defined in respective upper and lower surfaces thereof.
The dielectric strength of the electric double-layer capacitor is limited by the electrolytic voltage of the electrolytic solution 103. Therefore, depending on the dielectric strength which is required by the electric double-layer capacitor, a certain number of basic cells 106 are connected in series with each other, providing the cell stack 109. The terminal electrodes 108 are attached to the respective opposite ends of the cell stack 109 in the direction in which the basic cells 106 are stacked. The pressure plates 107 are attached to those surfaces of the terminal electrodes 108 which face away from the cell stack 109. Pressure plates 107 are pressed toward each other, and remain under a constant pressure for pressing the adjacent basic cells 106 against each other and also for pressing terminal electrodes 108 against adjacent basic cells 106 for thereby reducing contact resistances across contacting surfaces of the parts of the electric double-layer capacitor.
Polarizable electrodes 101, which have recently been employed in the art, are effective for increasing the capacity of the electric double-layer capacitor and reduce the equivalent series resistance of the electric double-layer capacitor. This increased capacity and reduced equivalent series resistance have led to new applications for the electric double-layer capacitor. Such new applications include a power supply for energizing an automobile starter motor in combination with a lead storage battery, and an auxiliary power supply combined with a solar cell.
In applications of the electric double-layer capacitor, the electric double-layer capacitor is highly likely to be installed in a high-temperature environment. Accordingly, it is necessary to keep the electric double-layer capacitor highly reliable in high-temperature usage. However, when a test is conducted in a high-temperature environment to evaluate the reliability of the conventional electric double-layer capacitor shown in FIG. 1, basic cells 106 are expanded because electrolytic solution 103 is expanded with heat and gases are produced from cell stack 109 by the application of a voltage. The effect which the expansion of the basic cells 106 has on each of the collectors 102 of the cell stack 109 is greater on the collectors 102 that are close to the ends of the cell stack 109 than on the collectors 102 in the central region of the cell stack 109. Particularly, the outermost collectors 102 are subject to the greatest load from the expansion of basic cells 106. Therefore, when the electric double-layer capacitor is placed in a high-temperature environment for a long period of time, the outermost collectors 102 tend to crack or break, allowing the electrolytic solution 103 to leak from the outermost basic cells 106.
In applications which require an electric double-layer capacitor to produce greater electric power, e.g., if an electric double-layer capacitor is used to energize an automobile starter motor, the electric double-layer capacitor needs to have a low equivalent series resistance. In order to reduce the equivalent series resistance of the electric double-layer capacitor shown in FIG. 1, the polarizable electrodes 101 and the collectors 102 may be electrically connected for good conductivity. For this reason, cell stack 109 is pressed by the pressure plates 107, each in the shape of a less deformable, highly rigid metal plate, applied to the outermost collectors 102 of cell stack 109.
However, collectors 102, which are made of electrically conductive rubber, are not rigid. Therefore, when cell stack 109 is pressed by pressure plates 107, collectors 102 are subjected to strong local forces. Polarizable electrodes 101 are made of sintered activated carbon, and hence are hard and highly rigid. Gasket frames 105 are often made of a hard material such as ABS resin so that they have high dimensional accuracy. Consequently, when cell stack 109 is pressed by metallic pressure plates 107 on the opposite ends of cell stack 109, outermost collectors 102 are liable to crack due to contact with sharp portions, such as edges, of polarizable electrodes 101 and gasket frames 105. As a result, when electric double-layer capacitors are manufactured, defective electric double-layer capacitors are produced with greater probability, and the productivity of electric double-layer capacitors is reduced.
An electric double-layer capacitor proposed in an attempt to solve the above problems is disclosed in Japanese Patent Laid-Open Publication No. 315191/93, for example. This disclosed electric double-layer capacitor comprises a cell stack of basic cells similar to basic cells 106 shown in FIGS. 1 and 2, with outermost collectors thicker than inner collectors.
Japanese Patent Publication No. 32529/92 discloses an electric double-layer capacitor comprising a cell stack of basic cells having collectors each in the form of a single electrically conductive polyethylene film. The electrically conductive polyethylene film is fabricated by filling carbon black with polyethylene at successively different densities. Specifically, the polyethylene in the electrically conductive polyethylene film is provided in three layers having successively low, high, and low densities across the thickness of the electrically conductive polyethylene film. Collectors thus constructed are mechanically strong and positioned as outermost collectors of the cell stack. Inasmuch as the strong outermost collectors are resistant to crack or breakage, the electrolytic solution is prevented from leaking out of the outermost basic cells.
Japanese Publication No. 36222/92 discloses an electric double-layer capacitor comprising a cell stack of basic cells having collectors each in the form of an electrically conductive sheet with a core disposed therein. Collectors thus constructed are mechanically strong and positioned as outermost collectors of the cell stack. The outermost collectors are thus resistant to crack or breakage, and the electrolytic solution is prevented from leaking out of the outermost basic cells.
However, the above conventional electric double-layer capacitors are disadvantageous in that it is difficult to increase the productivity of the electric double-layer capacitors and maintain the reliability of the electric double-layer capacitors. Reasons for these shortcomings will be described below.
According to Japanese Patent Laid-Open Publication No. 315191/93, the outermost collectors are thicker than the inner collectors of the cell stack, and the reliability of the electric double-layer capacitor is high because the mechanical strength of the outermost collectors is large. However, since each of the outermost collectors is in the form of a single sheet, if a crack or pinhole happens to be developed somewhere in any of the outermost collectors, then the crack or pinhole tends to be gradually enlarged in size, allowing the electrolytic solution to leak out of the corresponding outermost basic cell. Accordingly, the electric double-layer capacitor lacks a sufficient level of reliability. Another problem with the thicker collectors is that it is difficult to find any pinholes produced in the thicker collectors, resulting in an increased number of defective electric double-layer capacitors being produced.
According to Japanese Patent Publication No. 32529/92, an electrically conductive polyethylene film for use as a collector has a three-layer structure of carbon black filled with three layers of polyethylene having successively low, high, and low densities. Sulfuric acid is diffused at a low rate into the high-density layer of polyethylene. The low-density layers of polyethylene have soft surface layers which tend to have a low contact resistance. Even though the collector is of the three-layer structure, it is in the form of a single sheet, and is liable to start cracking from any region which is mechanically weaker than other regions. As a consequence, the electric double-layer capacitor does not have a sufficient level of reliability.
According to Japanese Utility Model Laid-open Publication No. 36222/92, each collector is in the form of an electrically conductive sheet with a core embedded therein for increased mechanical strength. The collector disclosed is problematic in that depending on the material of the core, the core may break the collector or degrade the characteristics of the electric double-layer capacitor. For example, if the core in the electrically conductive sheet comprises a metal foil, then since the metal foil is mechanically stronger than the electrically conductive sheet, the metal foil may possibly cause damage to the electrically conductive sheet when bending stresses, for example, are applied to the collector while the collector is being poorly handled. Though the metal foil itself has a low resistance, the contact resistance across the interface between the metal foil and a member of rubber or synthetic resin is high when the metal foil is superposed on the member of rubber or synthetic resin. As a result, the electric double-layer capacitor suffers an increase in the equivalent series resistance and hence fails to have good characteristics. When a pinhole or crack is developed in a surface of the electrically conductive sheet of a collector which faces the polarizable electrode, the electrolytic solution in the basic cell leaks through the pinhole or crack and reaches the metal foil in the electrically conductive sheet. The electrolytic solution then gradually corrodes the metal foil, and when a material produced by the corrosion of the metal foil enters the basic cell, it causes harmful reactions tending to emit gases. Therefore, the core of the collector tends to adversely affect the reliability of the electric double-layer capacitor. Even if the core embedded in the electrically conductive sheet is made of an electrically conductive synthetic resin, rather than metal, inasmuch as the material of the core is different from the material of the electrically conductive sheet, the contact resistance across the contacting surfaces of the electrically conductive sheet and the core increases. Consequently, the reliability of the electric double-layer capacitor remains relatively low.
It is therefore an object of the present invention to provide an electric double-layer capacitor which has separators, polarizable electrodes, and an electrolytic solution reliably sealed in gasket frames, includes collectors resistant to breakage and hence leakage of the electrolytic solution from basic cells, can be manufactured with high productivity, and is highly reliable when used in a high-temperature environment.
According to an aspect of the present invention, a gasket frame housing a separator and a pair of polarizable electrodes disposed in confronting relation to each other with the separator interposed therebetween has an opening sealed by a collector comprising two laminated collector sheets. Since there is almost no possibility that both the two laminated collector sheets of the collector will crack or break, it is possible to keep an electrolytic solution sealed, together with the separator and the polarizable electrodes, in the gasket frame and seal the opening in the gasket frame with the collector. The percentage of defective electric double-layer capacitors is lowered, and the productivity of electric double-layer capacitors is increased. Even when the temperature of the electric double-layer capacitor increases to expand the components in the gasket frame to such an extent that the collector is deformed, there is almost no possibility that both the two laminated collector sheets will crack or break. Consequently, inasmuch as the mechanical strength of the collector is high when the collector is deformed, the electric double-layer capacitor has a long service life and a high level of reliability.
According to another aspect of the present invention, each of the collectors on the outermost surfaces of a stack of basic cells comprises two laminated collector sheets. Each of the basic cells comprises a separator, a pair of polarizable electrodes disposed in confronting relation to each other with the separator interposed therebetween, a gasket frame accommodating the separator and the polarizable electrodes, and a collector sealing the opening in the gasket. Because there is almost no possibility that both the two laminated collector sheets of the outermost collectors will crack or break, the opening in the gasket frame is reliably sealed by the collector. Consequently, the productivity and reliability of electric double-layer capacitors are high. When the components in the basic cells are expanded in a high-temperature environment, the outermost collectors of the stack are deformed to the greatest degree between the collectors of the basic cells. However, since each of the outermost collectors comprises two laminated collector sheets, any possibility that all the laminated collector sheets will crack or break is almost nil. Therefore, the separator and the polarizable electrodes remain reliably sealed in each of the outermost basic cells of the stack in a high-temperature environment. In some applications, terminal electrodes are attached to the outermost ends of the stack, and the outermost collectors of the stack are pressed. Even in such a case, inasmuch as it is not likely for all the laminated collector sheets of the outermost collector to crack or break under the pressure, the productivity and reliability of electric double-layer capacitors are high.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.